Parses a search URL into its corresponding RSS URL, e.g. http://www.marktplaats.nl/z.html?query=raspberry+pi&categoryId=0&postcode=3501DD&distance=0 into http://kopen.marktplaats.nl/opensearch.php?pmin=&pmax=&q=raspberry+pi&pc=3501DD&d=0&ts=0&g=0.
Just do a search on Marktplaats and use the URL of the results page.